1. Field of the Invention
This invention relates to ion sources, particularly to an improved cold-cathode ion source for use in ion implantation systems.
2. Description of the Prior Art
With the advent of ion implantation processes for fabrication of semiconductor devices and related components, it has become desirable to improve presently known ion sources for inclusion in ion implantation systems suitable for use in manufacturing environments. One type of source that is useful for ion implantation is the Penning cold-cathode source, which typically comprises a hollow anode, a first cathode separated by an insulator from one end of the anode and having an extraction aperture therein, a second cathode separated by an insulator from the opposite end of the anode and having a gas entry port therein, and a magnet coil surrounding the anode. A plasma discharge maintained between the anode and the cathodes is contained by the magnetic field produced by the coil. The plasma discharge ionizes feed gas introduced through the entry port. External extraction electrodes form an ion beam from the resulting ions in the plasma discharge. Water cooling means are typically provided to remove heat generated by both the coil and the plasma discharge. A gaseous feed material, for example, boron trifluoride for boron ions or phosphine for phosphorus ions, is introduced through the gas entry port. Alternatively, an elemental solid feed material, for example, arsenic or antimony, is vaporized in an external oven connected by a passage to the gas entry port. It is also known to place an oven in the region of the plasma discharge. Typical prior art cold-cathode sources are described in an article by J. R. J. Bennett, "A Review of PIG Sources for Multiply Charged Heavy Ions," IEEE Transactions on Nuclear Science, Vol. NS-19, pp. 48-68, April 1972.
Ion implantation is a high-vacuum process, and servicing the ion source in an ion implantation system requires opening at least the source end of the system to atmosphere in order to gain access to the source, and then re-exhausting that part of the system. Such a procedure is time consuming and disruptive in a manufacturing environment, and it is clearly desirable to increase the time between such servicing episodes.
Two main lifetime-reducing difficulties are encountered with the cold-cathode source described above. First, material sputtered mainly from the cathodes by the plasma discharge builds up between the cathodes and the anode, causes short circuits, and changes the shape and stability of the plasma discharge. Second, when solid source material is used, vaporized solid feed material condenses at cooler points in the passage connecting the oven to the gas entry port, eventually blocking the passage. Other problems can result from the erosion of parts of the source by the sputtering effects of the plasma discharge, and the buildup of an insulating layer of condensed feed material on the anode. Such a buildup can increase the voltage required to strike an arc between the anode and cathodes, and, if heavy enough, can completely prevent striking the arc.
The magnitude of the beam current produced by an ion source is importantly related to the production rate of an ion implantation system containing the source; generally, the higher the beam current the less time needed to implant a given dose of ions in a batch of product. Unfortunately, operating an ion source at a higher beam current typically accelerates development of the lifetime-reducing difficulties noted above. Thus, it is doubly important to reduce the effects of those difficulties.